Recently, MIMO (multiple input multiple output) system is one of the hottest segments in the wideband wireless communication technology. The MIMO system means the system that can raise communication efficiency of data using multiple antennas. And, the MIMO systems can be implemented using such MIMO scheme as spatial multiplexing scheme and spatial diversity scheme in accordance with a presence or non-presence of the same data transmission.
The spatial multiplexing scheme means the scheme for transmitting data at high speed by transmitting different data via a plurality of transmitting antennas simultaneously without increasing a bandwidth. The spatial diversity scheme means the scheme for obtaining transmission diversity by transmitting the same data via a plurality of transmitting antennas. Space time channel coding is an example for the space diversity scheme.
Moreover, the MIMO technique can be categorized into an open loop scheme and a closed loop scheme in accordance with a presence or non-presence of feedback of channel information to a transmitting side from a receiving side. The open loop scheme may include one of BLAST scheme of detecting a signal and increasing an information size amounting to the number of transmitting antennas in a manner that a transmitting side transmits information in parallel and that a receiving side detects a signal using ZF (zero forcing) and MMSE (minimum mean square error) scheme repeatedly, STTC (space-time trellis code) scheme of obtaining a transmission diversity and coding gain using a new space region, and the like. And, the closed loop scheme may include one of TxAA (transmit antenna array) scheme and the like.
In a radio channel configuration, a fading effect is generated in a manner that a channel status irregularly changes in time and frequency domains. In order to reconstruct data transmitted from a transmitter and find out a correct signal, a receiver corrects a received signal using channel information.
A wireless communication system transmits a signal known to a transmitter and a receiver both and then finds out channel information using an extent of the signal that is distorted on being transmitted on a channel. In this case, this signal is called a reference signal (or a pilot signal) and ‘finding out channel information’ is called channel estimation. The reference signal does not contain data actually and has high output. In case that data is transmitted or received using multiple antennas, it may be necessary to obtain a channel status between each transiting antenna and each receiving antenna. Hence, the reference signal exists for each transmitting antenna.
Coordinated MIMO system is proposed to reduce inter-cell interference in a multi-cell environment. If the coordinated MIMO system is used, a user equipment may be jointly supported with data in common by multiple base stations (multi-cell base-station). And, in order to improve performance of system, each base station may be able to support at least one user equipments (MS1, MS2, MSK) simultaneously using the same frequency resource. Moreover, the base station may be able to perform SDMA (space division multiple access) based on channel state information between the base station and the user equipment.
In the coordinated MIMO system, a serving base station and at least one or more cooperative base stations are connected to a scheduler via backbone network. The scheduler may be activated in a manner of receiving feedback of channel information on channel states between the cooperative base stations and the user equipments (MS1, MS2, . . . MSK), which are measured by the base stations (BS1, BS2, . . . BSM) via the backbone network. For instance, information for the coordinated MIMO operation is scheduled for the serving base station and the at least one or more cooperative base stations by the scheduler. In particular, the scheduler directs instruction for the coordinated MIMO operation to each of the base stations.
CoMP has been proposed to reduce inter-cell interference and improve performance of a user equipment on a cell boundary in a multi-cell environment. Using CoMP system, a user equipment may be supported with data in common by a multi-cell base station. In particular, in a multi-cell environment, it may be able to improve communication performance of a user equipment using CoMP scheme. For this, accurate channel estimation is necessary based on a reference signal from the multi-cell base station.
Basically, cells of the related art generate reference signal sequences and patterns based on cell identifiers (IDs), respectively. However, in case of a coherent joint processing scheme capable of transmitting data simultaneously from multi-cell, this reference signal generation may cause degradation of communication performance.